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United States Patent |
5,202,686
|
Rapp
,   et al.
|
April 13, 1993
|
Infrared Fourier transformation spectometer with plural
analog-to-digital converters and interleaved amplification factors
Abstract
An infrared Fourier transformation spectrometer comprising a non-linear
analog-to-digital converter device having at least one amplifier and one
sample and hold circuit connected downstream thereof, as well as an an
analog-to-digital converter following the latter, wherein an input signal
to be converted is to be supplied to one input of the amplifier and the
gain of the input signal is a function of the magnitude fo the input
signal, and wherein the output signal of the analog-to-digital converter
is evaluated giving regard to the respective gain, is characterized by the
fact that at least two analog-to-digital converters are provided whose
outputs are connected to a first controllable switching arrangement for
supplying selectively the output signals of one of the said
analog-to-digital converters to another evaulation means and that the
input signal is supplied to each of the said analog-to-digital converters
amplified by a different amplification factor. This allows to achieve high
resolution by simple means.
Inventors:
|
Rapp; Norbert (Malsch, DE);
Blavier; Jean-Francois (Fleron, BE);
Simon; Arno (Karlsruhe, DE)
|
Assignee:
|
Bruker Analytische Messtechnik GmbH (DE)
|
Appl. No.:
|
602172 |
Filed:
|
October 23, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
341/139; 341/138; 341/141 |
Intern'l Class: |
H03M 001/18 |
Field of Search: |
341/138,139,140,141,155
|
References Cited
U.S. Patent Documents
4069479 | Jan., 1978 | Carpenter et al. | 341/139.
|
4129864 | Dec., 1978 | Carpenter et al. | 341/139.
|
4449120 | May., 1984 | Rialan et al. | 341/141.
|
4823129 | Apr., 1989 | Nelson | 341/139.
|
4926175 | May., 1990 | Ishizawa et al. | 341/131.
|
4999628 | Mar., 1991 | Kakubo et al. | 341/139.
|
Foreign Patent Documents |
0104333 | Jul., 1983 | EP.
| |
2331890 | Jun., 1973 | DE.
| |
3900247 | Jan., 1989 | DE.
| |
274671 | Dec., 1989 | DE.
| |
WO8706080 | Oct., 1987 | WO.
| |
1545653 | May., 1979 | GB.
| |
Primary Examiner: Williams; Howard L.
Attorney, Agent or Firm: Hackler; Walter A.
Claims
We claim:
1. Infrared Fourier transformation spectrometer comprising:
a non-linear analog-to-digital converter device having at least two
analog-to-digital converters having outputs connected to a first
controllable switching means for selectively supplying output signals of
one of the analog-to-digital converters to a calculating unit,
a plurality of amplifier means of different but constant amplification
factors disposed upstream of said analog-to-digital converters for
amplifying input to each of the analog-to-digital converters with
sequential gain factors alternating between said analog-to-digital
converters, said input signals being amplified by a constant but different
amplification factor;
a second controllable switching means for selecting one of the input
signals, which has been amplified by one of the plurality of amplifiers,
and
a sequence control means for switching said first controllable switching
means, said first controllable switching means being switched with a
certain time delay, when switching occurs, after switching of the second
controllable switching means.
2. A spectrometer according to claim 1, wherein conversion ranges of two
neighboring analog-to-digital converters have overlapping working ranges.
3. A spectrometer according to claim 1 wherein the sequence control means
is responsive to a value of the input signal as a function of time.
4. A spectrometer according to claim 1 wherein the sequence control means
controls the said first and, if necessary, also the second switching
device in response to a value and a curve of the input signals.
5. A spectrometer according to claim 1 wherein the sequence control means
controls the said first switching means and, if necessary, the said second
switching means in such a way that at any given time the output signals of
at least two neighboring analog-to-digital converters are
provided--neighboring as regards the working ranges--and are available for
optional selection by the first switching means.
6. A spectrometer according to claim 1, wherein the analog-to-digital
converters each have a resolution of at least 20 bits.
7. A spectrometer according to claim 1 wherein conversion ranges of two
neighboring analog-to-digital converters--neighboring as regards the
working ranges--overlap each other, and the sequence control means
controls the first and, if necessary, also the second controllable
switching means in response to the value of the input signal and time, at
least over part of the measurement and according to a fixed rule, and in
such a way that at any given time the output signals of at least two
neighboring analog-to-digital converters--neighboring as regards the
working ranges--are provided and are available for optional selection by
the first switching means, and that the analog-to-digital converters have
a resolution of at least 20 bits.
8. Infrared Fourier transform spectrometer comprising:
a non-linear analog-to-digital converter device having at least two
analog-to-digital converters, each preceded by a sample and hold circuit,
having outputs connected to a first controllable switching means for
selectively supplying the output signals of one of the analog-to-digital
converters to a calculating unit,
groups of amplifiers of different but constant amplification factors
disposed upstream of each of the analog-to-digital converters, and
second controllable switching means for selecting one of the output signals
of each group of amplifiers, for transmission to the analog-to-digital
converters with sequential gain factors of the amplifiers alternating
between the two analog-to-digital converters in the event that said second
switching means switches over, the first switching means will switch over
with a certain time delay, if at all.
9. Infrared Fourier transform spectrometer comprising:
a non-linear analog-to-digital converter device having two
analog-to-digital converters, each preceded by a sample and hold circuit,
having outputs connected to a first controllable switching means for
selectively supplying the output signals of one of the analog-to-digital
converters to a calculating unit,
groups of amplifiers of different but constant amplification factors
disposed upstream of each of the analog-to-digital converters, and
a second controllable switching means for selecting one of the output
signals of each group of amplifiers, for transmission to the
analog-to-digital converters with sequential gain factors of the
amplifiers alternating between the two analog-to-digital converters.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an infrared Fourier transformation
spectrometer comprising a non-linear analog-to-digital converter device
having at least one amplifier and one sample and hold circuit connected
downstream thereof, as well as an analog-to-digital converter following
the latter, wherein an input signal to be converted is to be supplied to
one input of the amplifier and the gain is a function of the magnitude of
the input signal, and wherein the output signal of the analog-to-digital
converter is evaluated giving regard to the respective gain.
Among the known infrared Fourier transformation spectrometers, there is for
example one offered by Bruker Analytische Me.beta.technik GmbH, D-7512
Rheinstetten, Federal Republic of Germany, as model IFS 88. An infrared
interferometer incorporated in the spectrometer supplies the described
input signal, namely an interferogram. The digital values supplied by the
converter device are processed by Fourier transformation by an arithmetic
unit, which is part of the spectrometer, in order to determine the
frequency fractions of the interferogram.
Converter devices of the before-mentioned type have been known in the form
of so-called floating-point analog-to-digital converters. In this case,
the input signal is amplified by a variable-factor amplifier in such a way
as to drive a subsequent fixed-point analog-to-digital converter module to
the maximum. One achieves in this way higher resolution, by the
amplification factor, and consequently a corresponding improvement of the
volume range. The input signal is sampled in a sample and hold circuit.
Thereafter, the optimum amplification factor is determined with the aid of
comparators, and the amplifier is adjusted correspondingly. Thereafter,
once the amplifier has assumed its steady-state condition, conversion can
proceed. Given the fact that the sample and hold circuit has only limited
resolution and holding capacity (pedestal error, drop rate), errors will
arise which are then amplified as well. Consequently, high quality is
needed for this sampling element. In addition, there may be cases where a
different gain is adjusted for each sampling operation, so that the
amplifier must assume its steady state very rapidly. The amplifier,
therefore, must have a very important band width, and the amplification
factors must be adjusted very precisely. Any balancing error will result
in deteriorated resolution. And the other demands placed on the amplifier
make it even more difficult to achieve high resolution.
DE-23 31 890 B2 shows a Fourier spectrometer comprising a single sample and
hold circuit and a single analog-to-digital converter. A first signal path
supplies signals characteristic of the position of the mirror, which are
supplied to a sample and hold circuit as control signals. The signal to be
sampled and to be converted is supplied via a different signal path.
EP-0 104 333 A2 describes an infrared Fourier transformation spectrometer
which is capable of converting two different analog signals to digital
values and of storing them practically simultaneously. This is achieved by
operating analog-to-digital converters alternately.
From DE-39 00 247 A1 it results that it has been known before in connection
with a Fourier spectrometer to have an analog-to-digital conversion
preceded by an amplification step.
SUMMARY OF THE INVENTION
Now, it is the object of the present invention to provide a spectrometer of
the type described above by means of which high resolution of the
analog-to-digital conversion of the interferogram can be achieved by
simple means. The invention achieves this object by the fact that at least
two analog-to-digital converters are provided whose outputs are connected
to a first controllable switching arrangement for supplying selectively
the output signals of one of the analog-to-digital converters to another
evaluation means, that the input signal is supplied to each of the
analog-to-digital converters amplified by a constant but different
amplification factor, that a plurality of amplifiers of different but
constant amplification factors are arranged upstream of at least one of
the analog-to-digital converters, that a second controllable switching
arrangement is provided for selecting one of the signals, which have been
amplified by the said plurality of amplifiers, for transmission to the
analog-to-digital converter, and that in the event the second switching
arrangement switches over, the first switching arrangement will switch
over with a certain time delay, if at all.
It may happen that a very strong input signal can be processed without any
amplification and that only certain weaker signal fractions need to be
amplified. In any such case, it may still be convenient for technical
reasons to guide the strong signal through an amplifier having the
amplification factor 1, and for the sake of simplicity this is exactly
what will be assumed hereafter.
One advantage of the invention is seen in the fact that the amplification
factor of the individual amplifiers need not be changed, but that instead
the amplifiers are permanently set to different amplification factors.
This avoids possible delays in the evaluation process caused by the time
required by an amplifier for assuming its steady-state condition after a
change of its amplification factor. Uniform phase characteristics can be
achieved for all amplifiers in a simple manner by the use of identical
amplifiers in which case the desired amplification factor is achieved by
the provision of subsequent different voltage dividers. Temperature
stability of the amplifiers can be achieved in a simple manner. As the
amplification factor need not be changed, any error resulting from
incorrect adjustments of the amplification factor can be avoided. Finally,
the invention makes it possible to create relatively simple converter
devices using commercially available analog-to-digital converters with,
for example, 14 or 16 bits, in which case the amplification leads to a
conversion range of, for example, 20 bits at a sampling frequency of, for
example, 20 Khz.
The evaluation of the output signals of the analog-to-digital converters
must give due consideration to the amplification factors by which the
signal supplied to the converter has been amplified.
Selection of the output signals of one particular of the analog-to-digital
converters can be effected very rapidly with the aid of a digital switch,
without giving rise to transient conditions which would then make it
necessary to wait until the device has assumed its steady state. In order
to minimize errors provoked by the analog-to-digital converters or by the
fact that the converter has an accuracy tolerance of +/-0.5 bit, the
output signal selected for further processing should advantageously be the
output signal of that converter which is driven to the maximum, i.e. which
is supplied with a maximum analog input signal, without however being
overloaded. In order to achieve this situation, i.e. that the converters
are driven to the maximum, the different amplification factors used for
the input signal should conveniently differ by a factor far smaller than
2.sup.n wherein n is the number of bits of the analog-to-digital
converter. Another advantage is seen in the fact that the number or
analog-to-digital converters can be kept smaller than the total number of
possible different amplification factors (including the amplification
factor 1, if used).
The signal connected by the second controllable switching device is an
analog signal. As in the event the second switching device switches over
the first switching device will switch over only after a certain time
delay, if at all, time is allowed for the circuitry connected downstream
of the second switching device (for example the sample and hold circuit)
to assume its steady state so that no errors will be caused by transient
conditions.
According to one embodiment of the invention, at least one of the
analog-to-digital converters is coupled to the output of one of the
amplifiers, without a second switching device connected between these two
elements. The arrangement is such that at least one other
analog-to-digital converter can be coupled to one of several amplifiers by
means of the before-mentioned second switching device. This leads to a
particularly favorable arrangement, as will appear from the following
description of another embodiment of the invention. In the case of this
other embodiment of the invention, the device comprises a group consisting
of two analog-to-digital converters and three amplifiers, one of the two
analog-to-digital converters of the group is coupled, without an
intermediate second switching device, to the output of that amplifier
whose amplification factor is between the amplification factors of the two
other amplifiers, and the other analog-to-digital converter can be coupled
selectively to one of the outputs of the two other amplifiers, via a
second controllable switching device. Consequently, the effective
amplification factor is not changed over for one of the analog-to-digital
converters, and by switching the effective amplification factor of the
other analog-to-digital converter between two values, in the described
simple manner, it is possible to cover an input signal range the
conversion of which requires the use of all the three amplification
factors. In addition, it is an advantage of this arrangement that there
are always simultaneously available the digital output signals of two
neighboring analog-to-digital converters, speaking in terms of the
conversion range, so that the selection as a function of the magnitude of
the input signal can be effected very rapidly and in a trouble-free
manner. There may also be provided more than one such groups in a device
according to the invention.
According to one embodiment of the invention, there are provided two
analog-to-digital converters, and each of the analog-to-digital converters
is preceded by a plurality of amplifiers having different constant
amplification factors. Consequently, this embodiment uses more than three
amplifiers. The advantages achieved in this case are the same as those
provided by the embodiment described immediately before.
The input signal supplied by the infrared interferometer, which comprises a
moving mirror, develop constantly over time, i.e. do not change suddenly.
The development over time of the described signal, as it results from the
mirror movement, is generally known. Consequently, there may be provided,
according to one embodiment of the invention, a sequence control for
controlling the first and, if necessary, also the second switching device
in response to the value of the input signal and the time, at least over
part of the measurement and according to a fixed rule. It is thus
possible, in particular, to follow a very rapid rise of the input signal.
According to one embodiment of the invention, a control device controls the
first switching means and the second switching means in such a way that at
any given time the output signals of at least two neighboring
analog-to-digital converters are provided--neighboring as regards the
working ranges--and are available for optional selection by the first
switching means. This permits rapid switching over to the output signals
of that analog-to-digital converter which is at that time driven to the
maximum.
The sequence control is, preferably, designed in such a way that, based on
general knowledge of the future signal curve, it will always switch on
that analog-to-digital converter which corresponds to the signal present
at any time and, in addition, that analog-to-digital converter (together
with the corresponding amplification factor for the input signal, if this
has to be selected) which will be needed the next in time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of one embodiment of an infrared Fourier
transformation spectrometer comprising an analog-to-digital converter
device;
FIG. 2 shows a sample curve of an input signal for the converter device;
and
FIG. 3 shows a modification of the arrangement illustrated in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, an infrared interferometer 1, which comprises a detector,
generates the signal to be converted at an input terminal 2 to which are
connected the input of a first operational amplifier OP1 and, via buffer
amplifiers TV, the inputs of two other operational amplifiers OP2 and OP3.
The operational amplifiers OP1 to OP3 have different amplification
factors, the amplification factor V1 of the first operational amplifier
OP1 being the smallest, the amplification factor V3 of the third
operational amplifier OP3 being the greatest, and the amplification factor
V2 of the second operational amplifier OP2 being in between. The buffer
amplifiers have an amplification factor of 1.
Each amplifier is followed by a low-pass filter 5 whose output signal is
freed from any DC fractions by an RC element. The outputs of the RC
elements assigned to the first and to the third operational amplifiers are
connected in this diagram to a controllable change-over switch 10 in such
a way that one of the two signals can be supplied selectively to the input
of a sample and hold circuit (S and H) 12 whose output is then connected
to the input of a first analog-to-digital converter 14. The output of the
RC element assigned to the second operational amplifier OP2 is directly
connected to the input of another sample and hold circuit 16 whose output
is connected to an additional analog-to-digital converter 18. The
before-mentioned sample and hold circuits and the analog-to-digital
converters, respectively, have all the same design. The outputs of the two
analog-to-digital converters 14 and 18 are connected to a calculating unit
20.
In order to avoid different phase-frequency characteristics, the
operational amplifiers OP1 to OP3 make all use of absolutely identical
amplifiers, the different amplification factors being obtained by
different voltage dividers arranged at the outputs of the individual
amplifiers.
While the RC elements remove all DC fractions, they let through the signals
received from the output of the low-pass filters. The low-pass filters
have the effect to reduce the band width of the signals to be processed
and, consequently, the noise level.
A control device designated as sequence control 30 is supplied with a clock
pulse. Control outputs of the sequence control control the switch 10, the
sample and hold circuits 12 and 16 as well as the analog-to-digital
converters 14 and 18. The calculating unit 20 sends different signals to
the sequence control. An EOC (end of conversion) signal informs the
sequence control that the signal supplied by the outputs of the
analog-to-digital converters has been recognized and evaluated so that the
sequence control 30 can, among other things, cause the sample and hold
circuits 12 and 16 to sample another value of their respective input
signals. In addition, the calculating unit 20 informs the sequence control
30 when an overflow has occurred for any of the analog-to-digital
converters, i.e. when the maximum conversion range of the respective
converter has been exceeded because its input signal has become
excessively high. The sequence control then switches over to the other
analog-to-digital converter which is not yet driven to the full. In
particular, the arrangement is such that when the signal amplitude rises
the amplification is reduced exactly at the moment when an overflow of the
previously active analog-to-digital converter has occurred.
Data losses are prevented by the fact that there are always operating
simultaneously the analog-to-digital converter with the previously optimum
amplification factor, which has been driven to the optimum up to that
moment, and another analog-to-digital converter with the next lower
amplification factor (for a higher input signal) which is assumed to be
required the next.
The sequence control 30 further sends signals to the calculating unit 20
which inform the latter which of the amplifiers is connected at any time
to the sample and hold circuit 12. In addition, the sequence control
supplies a switching signal to one of the digital switches contained in
the calculating unit 20, which switch will then, depending on its
switching position, connect the output signal of one exactly of the
analog-to-digital converters 14 and 18 to an output of the arithmetic unit
from where the signals will be transmitted to a computer 40 which will
perform a Fourier transformation and supply the result to an output unit
50, such as a display or a printer.
FIG. 2 shows an exemplary curve of a signal from the infrared
interferometer 1 which is to be converted by the converter device
contained in FIG. 1 (interferogram). The horizontal axis of FIG. 2 is the
time axis, while the signal amplitude in volts has been plotted against
the vertical axis. It can be seen that at the beginning the signal varies
slightly at an amplitude of less than 0.4 V, rises then very quickly to a
value of about 10 V, and drops thereafter again to very small values. The
waveshapes before and after the occurrence of the maximum signal amplitude
being very important for the Fourier transformation, it is necessary that
these curve portions be converted to digital values with high precision.
Given the fact that the general signal curve is known, the sequence
control of the illustrated example is designed in such a way that
initially the greatest amplification factor is active (the third
operational amplifier OP3 is switched on and the analog-to-digital
converter 14 is active), and as the magnitude of the signal supplied to
the analog-to-digital converter 14 rises, the system is switched over to
the analog-to-digital converter 18, which is coupled to the operational
amplifier OP2 immediately when the first-mentioned analog-to-digital
converter approaches its full-output limit. Thereafter, the switch 10 is
switched over by the sequence control as the signal continues to rise. As
soon as the analog-to-digital converter 18 approaches its upper
full-output limit, the analog-to-digital converter 14 is switched on
again, or the latter's output signals are selected so that the smallest
amplification factor becomes active for the signal.
When the signal magnitude drops, the system described by way of example
waits first to see if the signal will rise again within a short period of
time. Otherwise the system will switch over to the analog-to-digital
converter coupled to the amplifier having the next higher amplification
factor at the moment when the signal is in a magnitude range which
approaches the lower output limit of that analog-to-digital converter
whose output signal is just being evaluated, being already in the upper
range of the analog-to-digital converter coupled to the amplifier having
the next higher amplification factor (due to the fact that the conversion
ranges of the converters overlap each other).
During conversion of the analog signal illustrated in FIG. 2--viewed from
the left to the right--switching-over is effected (digitally) by the first
switching control from V2 to V1 (i.e. from the analog-to-digital converter
18 to the analog-to-digital converter 14) in order to accommodate the
signal peak. Thereafter (as the signal magnitude drops), the system is
switched-over again, digitally, to the converter coupled to the amplifier
OP2. If the signal amplitude continues to drop, the analog switch (second
switching means 10) switches over so that the analog-to-digital converter
14 is coupled to the amplifier OP3. Only when the signal remains within
the full-output range of the analog-to-digital converter 14 will the
system be switched over digitally to this latter converter with a certain
time delay permitting the system to assume a steady state.
By proceeding in this way, it is ensured that the signal being evaluated at
any time is always switched digitally and that sudden signal changes
caused by transient conditions will be avoided in that analog branch which
is active at any time. The demands placed on the rapidity by which analog
switching must be effected are, therefore, not critical and transient
conditions can be allowed to steady down. It is an advantage of the
described arrangement that the by far greatest part of the interferogram
is recorded by a single sample and hold circuit and a single
analog-to-digital converter.
The sequence control may be effected in a purely digital way as monitoring
the thresholds can be performed by means of the analog-to-digital
converter active at any time. Time monitoring is effected by the clock
pulse.
In the illustrated embodiment, the amplification factors are V1=1, V2=32,
V3=128. This corresponds to the binary exponents of 1, 5 and 7. The
analog-to-digital converters used in the example are model AD 1376
converters made by Analog Devices. The converters are of the 16 bit type
and accept input voltages of plus/minus 10 V. The converter characteristic
of the device according to FIG. 1 is linear by sections.
The sample and hold circuit may be integrated in the respective
analog-to-digital converter.
FIG. 3 shows a modification of the circuit illustrated in FIG. 1, the
modification differing from the arrangement of FIG. 1 only as regards the
area between the input terminal 2 and the sample and hold circuits (S and
H) 12 and 16, and the sequence control. Consequently, FIG. 3 shows
substantially only those circuit components which differ from those of
FIG. 1.
The arrangement illustrated in FIG. 3 uses a total of six operational
amplifiers OP1 to OP6, the operational amplifier OP1 having the smallest
amplification factor V1. The operational amplifier OP6 has the greatest
amplification factor V6, the amplification factors V2 to V5 of the
remaining operational amplifiers OP2 to OP5 being in between, in the order
of their identification numbers. The operational amplifiers OP1, OP3 and
OP5 are arranged in a group, and their output signals, which are passed
through a low-pass filter 5 and an RC element each, as viewed in FIG. 1,
can be selected individually for transmission to the sample and hold
circuit 12 by the upper part (as viewed in FIG. 3) of a controllable
change-over switch 110. Similarly, the operational amplifiers OP2, OP4 and
OP6 are arranged in a group the output signals of which can be similarly
selected, by the lower part of the switching-over device 110, for
transmission to the scanning and holding circuit 16 known from FIG. 1. The
selection is controlled by the sequence control 130. As, contrary to FIG.
1, the sequence control 130 actuates two individual switches in the
controllable switching-over device 110, the sequence control is indicated
by a reference numeral different from that of the sequence control 30 in
FIG. 1. For the rest, the operation of the sequence control is identical
to that of FIG. 1. If the input terminal 2 is supplied with an input
signal which rises from a lower to a higher value, then the sequence
control 130 will initially connect to the sample and hold circuit 12 the
operational amplifier OP1 having the lowest amplification factor. The
latter's signal, after conversion by the respective analog-to-digital
converter 14 (FIG. 1), is then selected by the first controllable
switching device 20 (FIG. 1) for further processing. As the input signal
rises, the output signal of the operational amplifier OP2 is selected next
for further processing; to this end, the switch illustrated in the lower
part of the switching-over device 110 has been coupled to the output
signal of the operational amplifier 2 already before the output signal of
the respective analog-to-digital converter 18 (FIG. 1) is selected for
further processing. If the signal continues to rise further, the upper
switch of the switching-over device 110 is connected to the operational
amplifier OP3 and then, i.e. when any transient conditions have steadied
down, the output signal of the analog-to-digital converter 14 can be
selected for further processing. As to the selection of the input signals
of the remaining operational amplifiers, this is effected in line with the
preceding description. It is understood that regarding the second group of
operational amplifiers it would well be possible to do for example without
the operational amplifier OP6 having the greatest amplification factor V6,
without any material changes to the general principle. But it seems
important that the amplification factor V2 be between V1 and V3, the
amplification factor V4 between V3 and V5, the amplification factor V3
between V2 and V4, and the amplification factor V5 between V4 and V6 (if
there is an OP6).
It will be readily appreciated by the man skilled in the art that the
principle illustrated in FIG. 3 enables more than six output signals of
operational amplifiers, or for example the output signals of only four
operational amplifiers, to be processed with the aid of two
analog-to-digital converters, in which case the upper and the lower parts
of the switching-over device 110 may select two output signals each.
The operational amplifiers of the arrangement illustrated in FIGS. 1 and 3
are designed in such a way that the reverse feedback gain adjusted in the
conventional manner is identical for all operational amplifiers OP1 to OP3
or OP1 to OP6, while the different gain is achieved, with otherwise
identical operational amplifiers, by different voltage dividers connected
to their outputs, this arrangement being not shown in the drawing for the
sake of simplicity.
The reference numerals used in the claims are not to be understood as
limiting the invention, but are only meant to simplify the understanding
thereof.
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